1. Field of the Invention
The present invention relates to a thin film circuit substrate comprising at least one conductor layer and one protecting layer formed on an electrically insulating substrate, and particularly relates to a thin film circuit substrate which is suitable for a thermal head.
2. Description of the Prior Art
Generally, a thermal head has such a structure that a glass glaze layer is partially formed on an electrically insulating substrate of alumina or the like, and, thereon, a heat-generating resistor layer, a conductor layer, an oxidization-proof layer, and a wear-proof layer are successfully formed. In this case, the oxidization-proof layer and the wear-proof layer are provided to protect the conductor layer. Such a thin film circuit substrate provided with a conductor layer and a protecting layer formed on an insulating substrate as described above is widely used in various fields.
Conventionally, in such a thin film circuit substrate of the kind described above, however, the surface accuracy of its insulating substrate has not been regarded as of major importance. Therefore, when a conductor layer 2 was formed on an insulating layer 1 and a protecting layer 3 was further formed thereon, the conductor layer 2 and the protecting layer 3 was not flat but zigzag with excessive ups and downs in section as seen in FIG. 2, due to unevenness of the surface of the insulating substrate 1. Therefore, pin holes 4 were considerably formed due to the zigzag portions in the protecting layer 3. Further, in the case of a thermal head, during fabrication erroneous contact with the human finger may occur in handling. Accordingly, there has been such problems that chlorine ions contained in sweat of a human body or the like comes into contact with the conductor layer 2 through the pin holes 4 and particularly in an atmosphere of high temperature and high humidity the conductor layer 2 is corroded to cause disconnection.